Field of the Technology
The present disclosure relates to methods for processing alpha-beta titanium alloys. More specifically, the disclosure is directed to methods for processing alpha-beta titanium alloys to promote a fine grain, superfine grain, or ultrafine grain microstructure.
Description of the Background of the Technology
Alpha-beta titanium alloys having fine grain (FG), superfine grain (SFG), or ultrafine grain (UFG) microstructure have been shown to exhibit a number of beneficial properties such as, for example, improved formability, lower forming flow-stress (which is beneficial for creep forming), and higher yield stress at ambient to moderate service temperatures.
As used herein, when referring to the microstructure of titanium alloys: the term “fine grain” refers to alpha grain sizes in the range of 15 μm down to greater than 5 μm; the term “superfine grain” refers to alpha grain sizes of 5 μm down to greater than 1.0 μm; and the term “ultrafine grain” refers to alpha grain sizes of 1.0 μm or less.
Known commercial methods of forging titanium and titanium alloys to produce coarse grain or fine grain microstructures employ strain rates of 0.03 s−1 to 0.10 s−1 using multiple reheats and forging steps.
Known methods intended for the manufacture of fine grain, very fine grain, or ultrafine grain microstructures apply a multi-axis forging (MAF) process at an ultra-slow strain rate of 0.001 s−1 or slower (see, for example, G. Salishchev, et. al., Materials Science Forum, Vol. 584-586, pp. 783-788 (2008)). The generic MAF process is described in, for example, C. Desrayaud, et. al, Journal of Materials Processing Technology, 172, pp. 152-156 (2006). In addition to the MAF process, it is known that an equal channel angle extrusion (ECAE) otherwise referred to as equal channel angle pressing (ECAP) process can be used to attain fine grain, very fine grain, or ultrafine grain microstructures in titanium and titanium alloys. A description of an ECAP process is found, for example in V. M. Segal, USSR Patent No. 575892 (1977), and for Titanium and Ti-6-4, in S. L. Semiatin and D. P. DeLo, Materials and Design, Vol. 21, pp 311-322 (2000), However, the ECAP process also requires very low strain rates and very low temperatures in isothermal or near-isothermal conditions. By using such high force processes such as MAF and ECAP, any starting microstructure can eventually be transformed into an ultrafine grained microstructure. However, for economic reasons that are further described herein, only laboratory-scale MAF and ECAP processing is currently conducted.
The key to grain refinement in the ultra-slow strain rate MAF and the ECAP processes is the ability to continually operate in a regime of dynamic recrystallization that is a result of the ultra-slow strain rates used, i.e., 0.001 s−1 or slower. During dynamic recrystallization, grains simultaneously nucleate, grow, and accumulate dislocations. The generation of dislocations within the newly nucleated grains continually reduces the driving force for grain growth, and grain nucleation is energetically favorable. The ultra-slow strain rate MAF and the ECAP processes use dynamic recrystallization to continually recrystallize grains during the forging process.
A method of processing titanium alloys for grain refinement is disclosed in International Patent Publication No. WO 98/17836 (the “WO '836 Publication”), which is incorporated by reference in its entirety herein. The method in the WO '836 Publication discloses heating and deforming an alloy to form fine-grained microstructure as a result of dynamic recrystallization.
Relatively uniform billets of ultrafine grain Ti-6-4 alloy (UNS R56400) can be produced using the ultra-slow strain rate MAF or ECAP processes, but the cumulative time taken to perform the MAF or ECAP steps can be excessive in a commercial setting. In addition, conventional large scale, commercially available open die press forging equipment may not have the capability to achieve the ultra-slow strain rates required in such embodiments and, therefore, custom forging equipment may be required for carrying out production-scale ultra-slow strain rate MAF or ECAP.
It is generally known that finer lamellar starting microstructures require less strain to produce globularized fine to ultrafine microstructures. However, while it has been possible to make laboratory-scale quantities of fine to ultrafine alpha-grain size titanium and titanium alloys by using isothermal or near-isothermal conditions, scaling up the laboratory-scale process may be problematic due to yield losses. Also, industrial-scale isothermal processing proves to be cost prohibitive due to the expense of operating the equipment. High yield techniques involving non-isothermal, open die processes prove difficult because of the very slow required forging speeds, which requires long periods of equipment usage, and because of cooling-related cracking, which reduces yield. Also, as-quenched, lamellar alpha structures exhibit low ductility, especially at low processing temperatures.
It is generally known that alpha-beta titanium alloys in which the microstructure is formed of globularized alpha-phase particles exhibit better ductility than alpha-beta titanium alloys having lamellar alpha microstructures. However, forging alpha-beta titanium alloys with globularized alpha-phase particles does not produce significant particle refinement. For example, once alpha-phase particles have coarsened to a certain size, for example, 10 μm or greater, it is nearly impossible using conventional techniques to reduce the size of such particles during subsequent thermomechanical processing, as observed by optical metallography.
One process for refining the microstructure of titanium alloys is disclosed in European Patent No. 1 546 429 B1 (the “EP '429 patent”), which is incorporated by reference herein in its entirety. In the process of the EP '429 patent, once alpha-phase has been globularized at high temperature, the alloy is quenched to create secondary alpha phase in the form of thin lamellar alpha-phase between relatively coarse globular alpha-phase particles. Subsequent forging at a temperature lower than the first alpha processing leads to globularization of the fine alpha lamellae into fine alpha-phase particles. The resulting microstructure is a mix of coarse and fine alpha-phase particles. Because of the coarse alpha-phase particles, the microstructure resulting from methods disclosed in the EP '429 patent does not lend itself to further grain refinement into a microstructure fully formed of ultrafine to fine alpha-phase grains.
U.S. Patent Publication No. 2012-0060981 A1 (the “U.S. '981 Publication”), which is incorporated by reference herein in its entirety, discloses an industrial scale-up to impart redundant work by means of multiple upset and draw forging steps (the “MUD Process”). The U.S. '981 Publication discloses starting structures comprising lamellar alpha structures generated by quenching from the beta-phase field of titanium or a titanium alloy. The MUD Process is performed at low temperatures to inhibit excessive particle growth during the sequence of alternate deformation and reheat steps. The lamellar starting stock exhibits low ductility at the low temperatures used and, scale-up for open-die forgings may be problematic with respect to yield.
It would be advantageous to provide a process for producing titanium alloys having fine, very fine, or ultrafine grain microstructure that accommodates higher strain rates, reduces necessary processing time, and/or eliminates the need for custom forging equipment.